Geothermal heat exchanger and heat pump circuit

ABSTRACT

A heat exchanger and heat pump circuit having an operating subcircuit and a pump down subcircuit each including a liquid line and a heat transfer tube. The heat transfer tube of the pump down subcircuit surrounds the liquid line of the operating subcircuit. During the heating cycle, the heat pump circuit circulates refrigerant through both subcircuits to abstract heat from the heat source/sink. However, during the cooling cycle, the heat pump circuit circulates refrigerant only through the operating subcircuit. Also during the cooling cycle, the heat pump circuit places the pump down subcircuit at low pressure to thermally insulate the operating subcircuit liquid line.

BACKGROUND OF THE INVENTION

The present invention relates to heat pumps, and more particularly to aheat exchanger and a heat pump circuit for a direct expansion heat pump.

Heat pumps have long been used as year-round air conditioning systemsthat operate in a heating cycle and a cooling cycle. Heat pumps aregenerally more efficient than conventional heating and cooling systemsbecause they transfer rather than create heat. The fundamentalprinciples of heat pump operation are simple. In the heating cycle, theheat pump draws heat from an outside heat source such as earth, air, orwater and transfers it to the conditioned space. In the cooling cycle,the heat pump abstracts heat from the conditioned space and dissipatesit into an outside heat sink.

In a conventional heat pump circuit, refrigerant is pumped through anoutdoor coil where ambient air either heats or cools the refrigerant.The heated or cooled refrigerant is pumped through an indoor coil toheat or cool the conditioned space. Experience has revealed that thistype of heat pump is relatively inefficient, largely because ambient airdoes not function as a stable heat source/sink. A number of "heatexchangers" have been developed to increase the efficiency of heat pumpsby utilizing the earth or outside water as the heat source/sink. Heatexchangers replace the conventional outdoor coil and can be buried inthe ground or submerged in a well, lake or river to facilitate heattransfer between the refrigerant and the heat source/sink.

Heat exchangers are available in a variety of designs. Among the mostpopular designs are "U" shaped and coaxial heat exchangers. A typical"U" shaped design includes a liquid line and a vapor line that areconnected to form opposite legs of a "U". In a conventional coaxialdesign, the liquid line extends coaxially into the center of a heattransfer tube. The end of the liquid line is open to allow refrigerantto flow between the line and tube. In the heating cycle, liquidrefrigerant flows into the liquid line where it receives heat from theheat source. The refrigerant evaporates and flows out of the heatexchanger through the vapor line or heat transfer tube. The vaporizedrefrigerant flows through an indoor coil where it condenses. The heatreleased in the coil during the phase change is passed into theconditioned space. The liquified refrigerant then flows back into theheat exchanger to repeat the cycle. In the cooling cycle, vaporizedrefrigerant enters the vapor line or heat transfer tube where itcondenses to transfer heat to the heat sink. The liquid refrigerantpasses through the liquid line into an indoor coil where the liquidrefrigerant evaporates by abstracting heat from the conditioned space.The vaporized refrigerant then flows back into the heat exchanger torepeat the cycle.

It is well known that there is a refrigerant imbalance between thecooling and heating cycles. During the cooling cycle, liquid refrigerantmust fill the entire liquid line before it returns to the circuit foruse. Consequently, a tremendous amount of liquid refrigerant is neededduring the cooling cycle. However, the heating cycle does not requiresuch a large volume of refrigerant because the vaporized refrigerantexpands quickly and rises, returning to the circuit for use. To overcomethe imbalance, some manufacturers provide the system with a refrigerantreceiver that stores the refrigerant during the heating cycle when it isnot needed. Refrigerant receivers increase the size and cost of thesystem. Alternatively, some systems include multiple heat exchangerssome of which are shut down during the cooling cycle. The refrigerantpassing through the shut-down exchangers during the heating cycle isthereby made available for use by the remaining exchangers during thecooling cycle. This type of system is relatively expensive tomanufacture and install. And finally, some systems include a controlsystem that drains refrigerant from the system during heating and returnit to the system during cooling. Again, the control system increases thesize and cost of the heat pump system.

During the cooling cycle, a significant amount of heat dissipates fromthe vapor line as the refrigerant condenses. Some of this heat istransferred to the liquid line where it heats the liquid refrigerantcausing it to vaporize or "flash off". This reduces the efficiency ofthe system. To overcome this problem, a variety of methods for thermallyinsulating the liquid line from the vapor line have been developed. Onesimple method is to increase the distance between the vapor line and theliquid line. A second method is to wrap the liquid line with insulation.A third method is to separate the vapor line and liquid line by avacuum. All of these methods increase the manufacturing and installationcosts of the system.

In addition, in northern climates there is disparity in the amount ofheat exchange area needed during the heating cycle and the coolingcycle. Some manufacturers have addressed this problem by providing thesystem with multiple heat exchangers. During the cooling cycle, some ofthe heat exchangers are shut-down to provide the appropriate heatexchange area.

SUMMARY OF THE INVENTION

The aforementioned problems are overcome by the present invention whichprovides a heat exchanger having a pump down subcircuit and an operatingsubcircuit. Both subcircuits include a liquid line serially connectedwith a heat transfer tube. The liquid line of the operating subcircuitextends through the heat transfer tube of the pump down subcircuit.During the heating cycle, both subcircuits operate to provide maximumheat transfer between the heat source/sink and the refrigerant. Duringthe cooling cycle, however, the pump down subcircuit is not used totransfer heat. Instead, the liquid refrigerant is drawn from thesubcircuit to leave a low density gas surrounding the liquid line of theoperating circuit.

In a preferred embodiment, the heat exchanger includes a continuousU-shaped outer tube that is divided into two heat transfer tubes by abypass seal fit within the tube. Separate liquid lines extend coaxiallyinto each heat transfer tube and connect to the bypass seal. The bypassseal interconnects each liquid line with the heat transfer tubesurrounding the other liquid line to create an operating subcircuitinterwoven with a pump down subcircuit. The pump down subcircuitconnects to a portion of the heat pump circuit that contains lowpressure during the cooling cycle. As a result, the circuit draws liquidrefrigerant from the pump down subcircuit during the cooling cycle.Consequently, during the cooling cycle, the pump down subcircuitcontains a low density gas that insulates the liquid line from the heatdissipating from the vapor line. Further, the refrigerant circulatingthrough the pump down subcircuit during the heating cycle is availablefor use by the operating subcircuit during the cooling cycle.

The present invention provides a simple and effective heat exchanger andheat pump circuit that balances the refrigerant requirements between theheating and cooling cycles. In addition, the present inventioneffectively insulates the liquid line of the operating subcircuit fromthe heat dissipated by the condensing vapor during the cooling cycle.Further, the outer tube can be continuous so that no joints are locatedwithin the heat source/sink. This increases the reliability of thesystem and facilitates a double wall construction.

These and other objects, advantages, and features of the invention willbe more readily understood and appreciated by reference to the detaileddescription of the preferred embodiment and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative diagram of a heat pump system according to apreferred embodiment of the present invention;

FIG. 2 is a schematic diagram of the heat pump circuit;

FIG. 3 is a perspective view of the heat exchanger showing operation inthe heating cycle;

FIG. 4 is a top sectional view of the heat exchanger taken along lineIV--IV of FIG. 3;

FIG. 5 is a perspective view of the heat exchanger showing operation inthe cooling cycle;

FIG. 6 is a partially sectional view of the heat exchanger and thebypass seal;

FIG. 7 is a partially sectional view of the heat exchanger and analternative bypass seal;

FIG. 8 is a perspective view of an alternative heat exchanger; and

FIG. 9 is a schematic diagram of an alternative heat pump circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A heat pump system constructed in accordance with a preferred embodimentof the present invention is illustrated in FIGS. 1 and 2, and generallydesignated 10. The heat pump system 10 operates to either cool or heat aspace 22 by transferring heat to and from an outside heat source/sink,such as the earth 38. The system 10 includes a generally conventionalheat pump circuit 12 having an air handler coil 14, a compressor 16, areversing valve 18; an accumulator 20, a number of manifolds 24, 26, and28, a number of heat exchangers 32a-d, and a network of conventionalrefrigerant lines 36 interconnecting the various components. Except asdescribed below, the operation and interrelationship of these componentsis generally well known to those skilled in the art. Accordingly, theindividual components will not be discussed in detail. However, ageneral summary of the function of each of these components will beprovided.

The compressor 16 pumps a refrigerant through the heat pump circuit 12.As the refrigerant circulates, it abstracts, releases, and caries heatthroughout the circuit. In the cooling cycle, the refrigerant abstractsheat from the conditioned space 22 and dissipates it into the earth 38.In the heating cycle, the refrigerant abstracts heat from the earth 38and releases it into the conditioned space 22. In a preferredembodiment, compressor 16, reversing valve 18, and accumulator 20 arecontained in a cabinet 35 located outside of space 22. However, thelocation of these components is not important. For example, any of thesecomponents can be located within space 22, and in many applications thecompressor and air handler coil can be an integral unit.

The air handler coil 14 is located in the conditioned space 22 totransfer heat between the refrigerant and the conditioned space 22. Inthe heating cycle, the air handler coil functions as a condensor wherevaporized refrigerant is cooled and changed into a liquid. During thephase change, the refrigerant releases a significant amount of heat intothe space 22. In the cooling cycle, the air handler coil functions as anevaporator where liquid refrigerant is heated and changed into a vapor.During the phase change, the refrigerant abstracts a significant amountof heat from the space 22.

The heat exchangers 32a-d are typically buried in the earth 38 orsubmerged in an outside water source such as a well, river, stream, orlake. The heat exchangers 32a-d transfer heat between the refrigerantand the heat source/sink, in this case the earth 38. In the heatingcycle, the heat exchangers function as an evaporator where liquidrefrigerant is heated and changed into a vapor. During the phase change,a significant amount of heat is abstracted from the earth 38. In thecooling cycle, the heat exchangers function as a condensor wherevaporized refrigerant is cooled and changed into a liquid. During thephase change, a significant amount of heat is dissipated into the earth38.

The reversing valve 18 switches the system 10 between the heating andcooling cycles. During the heating cycle, the reversing valve 18 directsthe refrigerant from the compressor 16 to the air handler coil 14. Fromthe air handler coil 14, the refrigerant flows to the heat exchangers32a-d through manifolds 24 and 26. The refrigerant then flows from theheat exchangers to the reversing valve through manifold 28. Thereversing valve 18 directs the returning refrigerant through theaccumulator 20 and back to the compressor 16. During the cooling cyclethe flow is essentially reversed. The refrigerant from the compressor 16is directed to the heat exchangers 32a-d via manifold 28. Therefrigerant flows from the heat exchangers 32a-d to the air handler coil14 through manifold 24. From the air handler coil 14, the refrigerantreturns to the reversing valve 18 which directs it to the accumulator 20and back to the compressor 16.

Accumulator 20 prevents liquid refrigerant from flowing into andpossibly damaging the compressor. As refrigerant enters accumulator 20,the liquid refrigerant falls into the reservoir where it is stored whilethe vaporized refrigerant is free to exit the accumulator and flow tocompressor 16.

In certain applications, it may be necessary to add a receiver (notshown) or other conventional component for storing excess refrigerantduring the heating or cooling cycle.

The manifolds 24, 26, and 28 are typically housed in a pit 34 locatedadjacent to the heat exchangers 32a-d. The manifolds 24, 26, and 28allow the desired number of heat exchangers 32a-d to be connected inparallel to the heat pump system 10.

The refrigerant line network is generally conventional except for thepump down circuitry described below. Suffice it to say that the networkis comprised of copper tubing or other suitable materials thatinterconnect the various circuit components in a conventional manner.The network includes line 100 interconnecting compressor 16 andreversing valve 18, line 102 interconnecting reversing valve 18 and airhandler coil 14, line 104 interconnecting air handler coil 14 andsplitter 128 (described below), lines 106 and 108 interconnectingsplitter 128 with manifolds 24 and 26, lines 110 and 112 interconnectingmanifolds 24 and 26 with heat exchanger 32, line 114 interconnectingheat exchanger 32 and manifold 28, line 116 interconnecting manifold 28and reversing valve 18, line 118 interconnecting reversing valve 18 andaccumulator 20, line 120 interconnecting accumulator 20 and compressor16, and line 122 interconnecting line 108 and line 102.

A pair of thermostatic expansion valves 40a-b are placed along line 104to meter the flow of refrigerant. The expansion valves 40a-b areconnected in parallel with a pair of one-way check valves 41a-b. Thecheck valves 41a-b direct refrigerant through expansion valve 40a duringthe heating cycle and expansion valve 40b during the cooling cycle.Alternatively, the expansion valves can be replaced by suitable fixedorifice metering devices, capillary tubes or the like. The type and sizeof the metering device will be selected to provide the appropriaterefrigerant flow rate.

Additionally, a solenoid-operated valve 124 is placed along line 108 toallow selective control over the flow of refrigerant through line 108.Operation of valve 124 is controlled by a conventional control system(not shown). A one-way check valve 126 is placed along line 122 toprevent the flow of refrigerant from line 102 to line 108.

Splitter 128 divides the flow of refrigerant between the operatingsubcircuit and the pump down subcircuit during the heating cycle. Liquidrefrigerant flows into splitter 128 from line 104 and out of splitter128 through lines 106 and 108. In a preferred embodiment, splitter 128is a conventional "T" joint.

The heat exchangers 32a-d are generally identical and, as noted above,are buried or submerged in a heat source/sink such as the earth, a well,a lake, a pond, or a stream. The heat exchangers 32a-d are connected inparallel to the circuit through manifolds 24, 26, and 28. The number ofheat exchangers 32a-d will vary from application to applicationdepending on the desired amount of heat transfer. Each heat exchanger32a-d includes an operating subcircuit 50 and a pump down subcircuit 60(see FIGS. 2, 3, and 5). Each subcircuit includes a heat transfer tube52, 62, connected in series with a liquid line 54, 64. The heat transfertube 62 of the pump down subcircuit 60 surrounds the liquid line 54 ofthe operating subcircuit 50.

In a preferred embodiment, the heat exchanger 32 includes a continuous,substantially U-shaped outer tube 42 that is divided into two discretesegments 44 and 46 by a bypass seal 70. The two segments 44 and 46function as heat transfer tubes 52 and 62. Obviously, the continuousouter tube 42 can be replaced by a jointed outer tube assembly. Theliquid lines 54 and 64 are of substantially smaller diameter than theouter tube 42, and extend substantially coaxially into opposite ends ofthe outer tube 42. Liquid line 54 is connected to manifold 24 via line110 and liquid line 64 is connected to manifold 26 via line 112 (seeFIG. 2). The bypass seal 70 interconnects each liquid line 54 and 64with the corresponding heat transfer tube 52 and 62, respectively. Theouter tube and liquid lines are preferably conventional copper tubing,however, a wide variety of conventional materials will suffice. The twoheat transfer tubes 52 and 62 are interconnected by a vapor line 74having a one-way check valve 75 that permits refrigerant to flow onlyfrom heat transfer tube 62 to heat transfer tube 52 (see FIG. 2). Thetwo heat transfer tubes 52 and 62 are connected to manifold 28 via line114. Alternatively, vapor line 74 and check valve 75 can be eliminatedand an additional manifold (not shown) can be installed in communicationwith heat transfer tube 62. In this alternative, a new vapor line andcheck valve would extend between the new manifold (not shown) andmanifold 28 to join the refrigerant from heat transfer tubes 52 and 62.

During use, it is possible for the outer wall 42 to fail allowingrefrigerant to leak into the environment. To protect the environmentfrom refrigerant leaks and to provide a method for identifying theseleaks, the heat exchanger 32 preferably includes a double-wallconstruction that allows the heat exchanger to vent to the atmosphere.The outer tube 42 is surrounded by a continuous secondary tube 72 thatvents above-ground. In a preferred embodiment, the secondary tube 72 isa polyethylene sheath fitted snugly around the outer tube 42. The openends of the secondary tube 72 remain above-ground so that refrigerantleaking from the outer tube 42 vents above-ground. The system 10 can beprovided with conventional apparatus (not shown) for sensing the flow ofrefrigerant from the secondary tube 72.

The heat exchanger 32 is preferably grouted to improve heat transferwith the earth 38. Grout 80 is well known to those skilled in the artand consequently will not be discussed in detail. Suffice it to say thatvolcanic clay or concrete grout is preferred.

As noted above, the bypass seal 70 is fitted within the outer tube 42 todivide the outer tube 42 into segments 44 and 46. The location of thebypass seal 70 can vary depending on the specific design of the heatpump circuit 12 and the heat exchanger 32. For example, the bypass seal70 can be moved to compensate for refrigerant imbalance between thecooling and heating cycles. In addition, the bypass seal 70 can belocated on the opposite side of the "U" or at the bottom of the "U" tofacilitate the return of oil to the compressor. As perhaps bestillustrated in FIG. 6, the bypass seal 70 includes a plug 74 and a pairof bypass openings 76a-b extending through the plug. The bypass seal 70is preferably manufactured from brass or copper, however, a variety ofmaterials will suffice including a wide range of elastomers. Each bypassopening 76a-b includes a circular end 78 and a half-moon end 81 (SeeFIG. 4). The liquid lines 54 and 64 enter opposite sides of the bypassseal 70 through circular ends 78. The lines 54 and 64 are brazed,soldered, compression fit, or otherwise secured to the bypass seal 70.Alternatively, if the liquid lines 54' and 64' are of sufficiently smalldiameter, the bypass openings can be a pair of through bores 76a-b' (SeeFIG. 7). In a preferred embodiment, a crimp ring 82 is crimped aroundthe outside of the outer tube 42 to secure the bypass seal 70 in place.As a first alternative, the bypass seal 70 can be secured in place byinduction brazing or soldering. As a second alternative, the bypass sealcan be held in place by the liquid lines and an O-ring that alsoprovides a seal between the plug and the outer tube 42. A variety ofother conventional methods can be used to secure the bypass seal 70 inplace.

OPERATION

The heat pump system 10 operates in either a heating cycle or a coolingcycle. During the heating cycle, solenoid valve 124 is open andreversing valve 18 is actuated to interconnect line 100 with line 102and line 116 with line 118. Compressor 16 pumps vaporized refrigerantthrough line 100 to reversing valve 18. The refrigerant passes from thereversing valve to the air handler coil 14 through line 102. In the airhandler coil, the vaporized refrigerant condenses into a high pressureliquid thereby releasing heat energy into space 22. The liquidrefrigerant flows from the air handler coil to the liquid line splitter128 through line 104. As the refrigerant moves through line 104 itpasses through check valve 41a and expansion valve 40a. Expansion valve40a meters the refrigerant to separate the high pressure side of thecircuit from the low pressure side of the circuit. The refrigerant thenflows through lines 106 and 108 into manifolds 24 and 26. Check valve126 prevents refrigerant from flowing through line 102 into line 122.The refrigerant flows from manifolds 24 and 26 to the heat exchanger 32through lines 110 and 112. If more than one heat exchanger is installed,the manifolds divide the refrigerant to flow in parallel through all ofthe heat exchangers. The low pressure liquid L flows into the liquidline 54 and 64 of both subcircuits 50 and 60. The liquid refrigerant Lflows from the liquid lines 54 and 64 into the heat transfer tubes 52and 62 where the refrigerant vaporizes thereby abstracting heat from theearth 38 (See FIG. 3). The vaporized refrigerant V flows from heattransfer tube 62 through vapor line 74 and check valve 75 where itunites with the vaporized refrigerant from heat transfer tube 52. Thevaporized refrigerant flows to manifold 28 via line 114. If more thanone heat exchanger is installed, the vaporized refrigerant from all ofthe heat exchangers will return to manifold 28. From manifold 28, therefrigerant flows to the reversing valve 18 through line 116 and then tothe accumulator 20 through line 118. The refrigerant then flows back tothe compressor 16 via line 120 to complete the circuit.

During the cooling cycle, solenoid valve 124 is closed and the reversingvalve 18 is actuated to interconnect line 100 with line 116 and line 102with line 118. The compressor 16 pumps vaporized refrigerant throughline 100 to reversing valve 18. The reversing valve directs therefrigerant to manifold 28 via line 116. From manifold 28, the vaporizedrefrigerant V flows through line 114 into heat transfer tube 52. Checkvalve 75 prevents the refrigerant from entering heat transfer tube 62.The vaporized refrigerant V condenses in heat transfer tube 52 and theresulting liquid L is forced up through liquid line 54 (See FIG. 5). Theliquid refrigerant flows to manifold 24 through line 110. From manifold24, the liquid refrigerant flows to the air handler coil 14 through line106, liquid line splitter 128, and line 104. Solenoid valve 124 isclosed to prevent refrigerant from flowing through line 108. As therefrigerant moves through line 104 it passes through check valve 41a andexpansion valve 40b. Expansion valve 40b meters the refrigerant toseparate the high pressure side of the circuit from the low pressureside of the circuit. Inside the air handler coil 14, the refrigerantvaporizes thereby abstracting heat from space 22. The vaporizedrefrigerant then flows from the air handler coil to the reversing valve18 via line 102. The reversing valve directs the refrigerant to theaccumulator 20 through line 118. And finally, the refrigerant flows backto the compressor via line 120. Simultaneously, the pump down subcircuit60 is "pumped down" to insulate liquid line 54 from the heat dissipatedduring condensation of the vaporized refrigerant in heat transfer tube52. The low pressure in line 102 draws refrigerant from the pump downsubcircuit via lines 122, 108, and 112, leaving only low densityvaporized refrigerant. The refrigerant drawn from the pump downsubcircuit can be used by the circuit to overcome the refrigerantimbalance.

As noted above, the double wall construction protects the environmentfrom leaking refrigerant. If at any time the outer tube 42 begins toleak, the escaping refrigerant will be trapped between the outer tube 42and the secondary tube 72. The trapped refrigerant will flow between thetwo tubes until it reaches either or both open ends of the secondarytube 72. A leak can be detected by a sensing device or by visualinspection of the receptacle.

First Alternative Embodiment

In a first alternative embodiment, the heat exchanger is modified tolocate both liquid lines in a single heat transfer tube. As illustratedin FIG. 8, liquid line 54 is replaced by liquid line 54". Liquid line54" includes an open end 55 that communicates with heat transfer tube52". The bypass seal 70" is modified to include only a single bypassopening 76a" adapted to connect liquid line 64" with heat transfer tube62". Operation of the alternate heat exchanger 32" is generallyidentical to that of the preferred embodiment. During the heating cycle,refrigerant circulates through both the pump down and operatingsubcircuits. However, during the cooling cycle, the pump down subcircuitis pumped down to insulate liquid line 64" from heat dissipated duringcondensation of refrigerant in heat transfer tube 62".

Second Alternative Embodiment

A second alternative embodiment is shown in FIG. 9. In this embodiment,the heat pump circuit 12' is modified to return the liquid refrigerantdrawn from the pump down subcircuit 60 to the circuit upstream from theair handler coil 14. As illustrated, solenoid valve 124, splitter 128,line 106, and line 108 are eliminated. These components are replaced bya single line 104' extending between air handler coil 14 and manifold24, a single line 122' extending between manifold 26 and line 104'(between expansion valve 40b and air handler coil 14), and expansionvalve 130 placed along line 122' in parallel with check valve 126 toseparate the high and low pressure sides of the circuit. During thecooling cycle, refrigerant is free to flow out of the pump downsubcircuit 60 through check valve 126. During the heating cycle, theflow of refrigerant is reversed and the refrigerant flows into the pumpdown subcircuit through expansion valve 130.

The present invention is described in conjunction with a spaceconditioning system. Those skilled in the art will readily appreciateand understand that the present invention is equally well suited for usewith water heating systems, heat disposal systems, and other similarheating and/or cooling systems.

The above description is that of a preferred embodiment of theinvention. Various alterations and changes can be made without departingfrom the spirit and broader aspects of the invention as defined in theappended claims, which are to be interpreted in accordance with theprinciples of patent law including the doctrine of equivalents.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A heat exchanger for use with a heat pump, comprising:an operating subcircuit having a liquid line and a heat transfer tube; and a pump down subcircuit having a liquid line and a heat transfer tube, said liquid line of said operating subcircuit passing through said heat transfer tube of said pump down subcircuit, wherein neither of said heat transfer tubes passes through the other so that both of said heat transfer tubes define a heat transfer boundary with a common external heat transfer medium permitting both of said heat transfer tubes to exchange heat with said common external heat transfer medium.
 2. The heat exchanger of claim 1 wherein said heat transfer tube of said operating subcircuit and said heat transfer tube of said pump down subcircuit are defined by a single, continuous outer tube.
 3. The heat exchanger of claim 2 further comprising a bypass seal fitted within said outer tube to separate said heat transfer tube of said operating subcircuit from said heat transfer tube of said pump down subcircuit.
 4. The heat exchanger of claim 3 wherein said heat transfer tube of said operating subcircuit surrounds said liquid line of said pump down subcircuit.
 5. The heat exchanger of claim 4 wherein said outer tube is substantially U-shaped and includes a bend; andwherein said bypass seal is located adjacent said bend.
 6. The heat exchanger of claim 5 further comprising a secondary tube surrounding said outer tube.
 7. A heat exchanger comprising:a first subcircuit having a conduit in communication with a heat transfer tube; a second subcircuit having a conduit in communication with a heat transfer tube, said conduit of said first subcircuit passing through said heat transfer tube of said second subcircuit; wherein neither of said heat transfer tubes passes through the other so that both of said heat transfer tubes define a heat transfer boundary with a common external heat transfer medium permitting both of said heat transfer tubes to exchange heat with said common external heat transfer medium.
 8. The heat exchanger of claim 7 wherein said heat transfer tube of said first subcircuit and said heat transfer tube of said second subcircuit are defined by a single, continuous outer tube.
 9. The heat exchanger of claim 8 further comprising a bypass seal fitted within said outer tube to separate said heat transfer tube of said first subcircuit from said heat transfer tube of said second subcircuit.
 10. The heat exchanger of claim 8 wherein said heat transfer tube of said first subcircuit surrounds said conduit of said second subcircuit.
 11. The heat exchanger of claim 9 wherein said outer tube is substantially U-shaped and includes a bend; andwherein said bypass seal is located adjacent said bend.
 12. The heat exchanger of claim 10 further comprising a secondary tube surrounding said outer tube.
 13. A heat exchanger comprising:an outer wall defining an interior and having a longitudinal extent; a bypass seal fitted within said outer wall dividing said interior of said outer wall along its longitudinal extent into distinct first and second heat transfer tubes: a first conduit extending longitudinally through said second heat transfer tube, said first conduit being in communication with said first heat transfer tube; and a second conduit extending longitudinally through said first heat transfer tube, said second conduit being in communication with said second heat transfer tube.
 14. The heat exchanger of claim 13 wherein said outer tube is substantially U-shaped and includes a bend; andwherein said bypass seal is located adjacent said bend.
 15. The heat exchanger of claim 14 further comprising a secondary tube surrounding said outer tube. 